1. Field Of The Invention
The invention relates to a new method of preparing microcrystalline titanium dioxide, having the average crystal size &lt;100 nm, in which a) the solid titanium dioxide hydrate is treated with a base, b) the precipitate treated with the base is treated with hydrochloric acid, and c) the precipitate treated with the base and the hydrochloric acid is neutralized.
2. Description Of The Related Art
The properties of the microcrystalline titanium dioxide differ from the properties of the usual titanium dioxide known as white pigment. The differences are due to the dissimilarities in the crystal size, for the crystal size of the microcrystalline titanium dioxide (10-100 nm) is about 5 to 10 times smaller than the crystal size of the normal titanium dioxide (160 to 250nm). As the crystal size becomes smaller the coverage of titanium dioxide in the range of visible light disappears and the pigment becomes transparent. On the other hand, the permeability of UV-light decreases. Accordingly, the microcrystalline titanium dioxide of the present invention is suitable for a UV-protective agent. Thanks to the small crystal size and the great specific surface area the titanium dioxide of the invention can be used as an effect pigment among other things in catalysts, ceramics and paints.
The preparation of microcrystalline titanium dioxide needs a process of its own in which the difficulty lies in the creation, controlling and retaining of the small particle size throughout the process. The control of the purity and the crystal size distribution are important factors when assessing the various methods of preparation. Moreover, the preparation processes must be economically profitable and ecologically beneficial. Different crystal forms can be achieved by different methods. Rutile is known as a durable crystal form and its permeability to UV-light is less than that of anatase.
It is possible to prepare microcrystalline titanium dioxide by many different methods either by gas phase or by precipitation techniques.
The titanium source or the titanium containing material can be, e.g., titanium tetrachloride, titanium alcoxide or titanium hydrate prepared from ilmenite. For the preparation of rutile, a variety of precipitation methods have been patented. In the JP patent 86/049250 a method of preparing microcrystalline titanium dioxide, after-treated with aluminum and/or silicon dioxide, from titanium hydrate prepared from ilmenite and ammonia has been briefly presented. By using ammonia the formation of foreign metal cations into the pigment is avoided but, on the other hand, one is compelled to handle troublesome ammonia reagent. The crystal form of product is not mentioned in the patent specification, but it is not necessarily rutile. In a later application JP 57/67681 of the same Japanese applicant a similar method of preparation is presented by which finely-divided anatase created. Rutile is prepared in the same patent with a recipe starting from titanium tetrachloride. The neutralization is carried out with NaOH and water additions. The precipitation concentration is 30 g/liter TiO.sub.2.
In DE 3817909 preparation of microcrystalline titanium dioxide both from ilmenite and titanium tetrachloride is presented. The titanium hydrate mass prepared from ilmenite is treated with sodium hydroxide to form a titanium hydrate cake containing sodium. The pH is first set to the value of 2.8 to 3.0 with hydrochloric acid addition and at a later stage of the cooking the ratio between the acid and the titanium dioxide is adjusted to the value of 0.26. At the end of the cooking, a slurry is neutralized to the pH-value 7.5, after which the titanium dioxide is filtered and washed. Then the pigment is sand ground and after-treated with aluminum and/or silicon dioxide. Crystals of the microcrystalline titanium prepared in this way become acicular. A drawback of the method is that the pigment is not calcinated, whereby the crystals would become rounder and the crystal size could be regulated. A further drawback of this known method is that the hydrochloric acid amount which is added to create the rutile crystals is dependent on the titanium dioxide concentration.
Washing of the titanium dioxide so as to free it from precipitation salts is rather laborious, as the final neutralization is carried out at a pH-value of 7.5.
The same drawbacks also apply for the preparation of microcrystalline titanium dioxide from titanium tetrachloride presented in the same DE application.
In CA patent specification 962,142, a very similar method of preparation is disclosed as in the above-mentioned DE publication in which the starting material is ilmenite but after the neutralization of the precipitation (with ammonia to the pH-value 6.5) the composition is cooked before the filtration and the wash and the so-called calcination chemicals (K.sub.2 O, P.sub.2 O.sub.5) are added and the titanium dioxide is calcinated. This method does not produce titanium dioxide which is sufficiently microcrystalline, instead the crystal size achieved is 50 to 150 nm.
In the method presented in the JP patent application 59223231, in which the starting material is titanium hydrate mass, acicular rutile crystals are produced, which are coated with organic aluminum compounds. The titanium dioxide is not calcinated, for which reason regulation of the crystal size is troublesome. The filtration difficulties are overcome by means of an organic after-treatment. Accordingly, no conventional inorganic treatment recipes can be applied.
In the corresponding method of preparation starting from the titanium hydrate mass JP publication 62/235215 suggests solving the filtration and drying problem by means of a filtration film of .ANG. and freeze drying. The method is troublesome, prone to disturbances and unnecessarily expensive.